High Temperature Analysis of Si, SOI & SiC Piezoresistive Pressure Sensors

High Temperature Analysis of Si, SOI & SiC Piezoresistive Pressure Sensors

Kirankumar B. Balavalad

Electronics and Communication Engineering, Basaveshwar Engineering College, (Affiliated to VTU, Belagavi)

Bagalkot-587103, Karnataka, India

Vinayak. Lambi

Electronics and Communication Engineering, Basaveshwar Engineering College, (Affiliated to VTU, Belagavi)

Bagalkot-587103, Karnataka, India.

Abstract The paper discusses the high temperature performance of Silicon, Silicon on Insulator and Silicon Carbide based piezoresistive pressure sensors over the temperature range of 0 to 10000C. Junction leakage current is considered as one of the important parameters in this study and all the three sensors are analyzed for the junction leakage current. Simulation study reveals that the SiC based sensor has least temperature sensitivity compared to the other two sensors. In the design of SOI and SiC sensors the insulation layer is used between piezoresistors and the substrate. Junction leakage current is very small in case of SiC sensor. SiC sensor can be appropriate design for high temperature applications as SiC piezoresistors are very less sensitive to temperature.

Keywords SOI, SiC, piezoresistive pressure sensors. high tempearature, junction leakage current.

1. INTRODUCTION

   MEMS is defined as the combination of electrical and mechanical components. The advancement in MEMS technology is because of its features such as small in size, low cost, batch fabrication and consumes low power. MEMS pressure sensors are broadly used in many applications. These pressure sensors are characterized based on sensing mechanism. Absolute, gauge and differential pressure sensor is three main MEMS pressure sensor [1]. Piezoresistive pressure sensor is categorized under differential pressure sensor. Silicon based piezoresistive pressure sensor are widely used in commercial and industrial applications. But recently emphasis is given on piezoresistive pressure sensor which is used in harsh environment where the temperature goes up to 3000C [2]. Piezoresistive Pressure Sensors (pzr) are frequently used in chemical processing, aerospace, and automobile for pressure measurements. Piezoresistive materials are very much sensitive to the effect of temperature. As temperature changes the resistivity of the pzr material also changes [3]. A four piezoresistors connected in Wheatstone bridge configuration on a top of single crystalline Silicon substrate develops a residual stress. Such types of sensor have zero offset error which keeps on increasing as temperature increases [4]. A p-type piezoresistors on n-type substrate forms a junction. A leakage current is observed at this junction and it increases with respective to increase in the temperature. The leakage current affects the performance of the sensor. The temperature Coefficient of Resistivity (TCR) is a vital parameter in piezoresistive pressure sensor and it is always positive in single diffused crystal resistor. The sensitivity of the sensor is affected by TCR of piezoresistors which leads to Temperature Coefficient Sensitivity (TCS).

   The TCS gets negative as TCR increases [5]. By careful selecting the materials and tailor-made fabrication process we can avoid residual stress. By using an extra resistor on the diaphragm and use of double wheat stone bridge the temperature effect and zero pressure offset can be reduced [6- 9]. Poly Silicon material can withstand at high temperature but piezoresistivity effect over the oxide layer is small [5].

   [10] describes about Silicon on Insulator and its advantage over bulk silicon. SOI can provide stable operation at high temperature exceeds 125 C. It consists of buried oxide layer which can be used as insulating layer between the substrate and the piezoresistors [11-15]. The effect of leakage current can be reduced by using oxide layer [19].

2. DESIGN AND SIMULATION OF SI, SOI AND SIC

   The pzr sensor consists of four piezoresistors which are connected using connector line in the form of Wheatstone bridge configuration. R1, R2, R3 and R4 are the piezoresistors which are placed on the diaphragm as shown in fig 2.1. Here the diaphragm is used as sensing element. Piezoresistors are placed in high stress region.

   Fig 2.1 Proposed model of PZR sensor.

   The dimension of the diaphragm for proposed model is length of 400m, width of 400 m and height of 10 m. Dimension of Piezoresistors is length of 100 m, width of 10 m and thickness of 5 m. The dimension of diaphragm and piezoresistors remains same for all three models. For SOI and

   SiC model the oxide layer is used which has thickness of 2 m.

   TABLE 2.1. DIMENSIONS FOR ALL THE THREE SENSOR MODELS

   NAME	TYPE	LENGTH	WIDTH	THICKNESS
   Substrate	n-type silicon	500m	500m	40m
   Diaphragm	n-type silicon	400m	400m	10m
   Oxide	SiO2	500m	500m	2 m
   Piezoresistors	p-type	140m	10m	5m

   Table 2.1 shows the dimensions for all three models [17]- [19]. But in SiC based model, we use SiC material for piezoresistors and rest of the things remains same.

   MATERIAL PROPERTIES	N-TYPE SILICON	P-TYPE SILICON	SIO2
   Youngs Modulus	160e9	160e9	170e9
   Poissions Ratio	0.22	0.22	0.17
   Density (Kg/m3)	2330	2330	2200

   MATERIAL PROPERTIES	N-TYPE SILICON	P-TYPE SILICON	SIO2
   Youngs Modulus	160e9	160e9	170e9
   Poissions Ratio	0.22	0.22	0.17
   Density (Kg/m3)	2330	2330	2200

   TABLE 2.2 MATERIAL PROPERTIES OF ALL THREE MODELS.

   A pressure is applied on the diaphragm it starts to deform and displacement can be calculated using equation 1.

   2 4

   2 4

   D = 0.01512(1V )Pa (1)

   Ep

   Where, P is applied pressure, a is side length of the diaphragm

   E is Youngs modulus of silicon, h is Thickness of the diaphragm, v is poisons ratio of silicon. The output voltage depends on input voltage and pressure applied to the device. It can be measured by using equation 2.

   Fig 2.2 Sensor model after Meshing.

3. RESULTS AND DICUSSION

   In this segment the simulation results of Si, SOI and SiC models is discussed. Output parameter such as displacement, output voltage and leakage current are discussed. Pressure ranging from 0 kPa to 1000 kPa is applied to all three models.

   3. 1 Displacement and output voltage analysis

   Fig 3.1, Fig 3.2 and Fig 3.3 shows displacement of all the models. From the observation displacement of Si based pzr sensor is more as compared to SOI and SiC based pzr sensor.

   = (1)

   2

   2

   2

   (2)

   Where, t = Piezoresistive coefficient for inverse position. Te temperature sensitivity of piezoresistive pressure sensor is defined as the ratio of output voltage to the change in the temperature.

   = 1

   T Vin

   (3)

   Where, S is Sensitivity of the pressure, Vis change in output voltage, P is change in applied pressure and Vin is input Voltage to the pressure. The reverse leakage current is given by using equation (4)

   = + ) (4)

   = + ) (4)

   (

   Where, A is cross sectional area of diaphragm, q is charge, Dn and Dp are diffusion coefficients of minority carriers, Wp and Wn are width of p-type and n-type Silicon, npo and pno are equilibrium concentration of minority carriers. Fig 2.2 Shows meshing of proposed model, user-controlled mesh is used to perform meshing.

   Fig 3.1 Deformation of Si based pzr sensor.

   Fig 3.2 Deformation of SOI based pzr sensor.

   Fig 3.3 Deformation of SiC based pzr sensor.

   The results for the output potential for both the sensors are presented in Table 3.1, 3.2 & Table 3.3. The same is plotted in Fig. 3.4.

   Table 3.1 Simulated displacement and output voltage of Si based PZR sensor

   Applied Pressure (KPa)	Electric Potential(mV)
   0	374
   100	414.53
   200	454.59
   300	494.19
   400	533.31
   500	572.01
   600	610.26
   700	648.09
   800	685.5
   900	722.51
   1000	759.12

   Table 3.2 Simulated displacement and output voltage of SOI based PZR sensor.

   Applied Pressure (KPa)	Electric Potential(mV)
   0	374.08
   100	397.79
   200	421.34
   300	444.71
   400	467.94
   500	490.99
   600	513.88
   700	536.62
   800	559.2
   900	581.64
   1000	603.91

   Table 3.1 Simulated displacement and output voltage of Si based PZR sensor

   Applied Pressure (KPa)	Electric Potential(mV)
   0	2354.88
   100	2376.50
   200	2398.00
   300	2419.37
   400	2440.64
   500	2461.78
   600	2482.80
   700	2503.71
   800	2524.50
   900	2545.19
   1000	2565.77

   Fig 3.4 Applied Pressure v/s Electric Potential

   Figure 3.5. shows Temperature versus Leakage current graph. Leakage current values 33.655A to 96.95A for SOI based Piezoresistive Pressure Sensor and 33.66A to 96.96 A for

   SiC based Piezoresistive Pressure Sensor and also for 39.077A to 110.55 A Si based Piezoresistive Pressure Sensor when temperature ranges from 0C to 300C with applied pressure is 1000KPa which is constant for all temperature values. The graph indicates as temperature increases the leakage current also increases. But the effect of temperature on the output of sensor with SiO2 layer is much small compared to the sensor without SiO2 layer. And hence SiC based sensor shows very less junction leakage current & therefore less temperature sensitivity.

   Table 3.4 Simulated Leakage Current values of SOI, SiC and Si based PZR sensor

   Temperature (In degrees Celsius)	SOI based pzr Leakage Current (A)	SiC based pzr Leakage Current (A)	Si based pzr Leakage Current (A)
   0	33.65	33.66	39.07
   100	42.96	42.97	51.30
   200	59.44	59.45	75.06
   300	96.95	96.96	110.05

   Fig 3.5 Temperature V/S Leakage Current

   Temperature (In degrees Celsius)	Si based pzr Electric Potential (mV)	SOI based pzr Electric Potential (mV)	SiC based pzr Electric Potential (mV)
   0	759.52	604.43	2565.84
   200	757.77	604.05	2566.15
   400	751.33	594.20	2567.30
   600	744.94	586.21	2568.46
   800	738.39	578.03	2569.66
   1000	731.4	569.27	2570.96

   Temperature (In degrees Celsius)	Si based pzr Electric Potential (mV)	SOI based pzr Electric Potential (mV)	SiC based pzr Electric Potential (mV)
   0	759.52	604.43	2565.84
   200	757.77	604.05	2566.15
   400	751.33	594.20	2567.30
   600	744.94	586.21	2568.46
   800	738.39	578.03	2569.66
   1000	731.4	569.27	2570.96

   Table3.7 . Simulated values of electric potential for Silicon, SOI and SiC Based PZR Sensors

   Fig 3.6. shows Temperature v/s Electric Potential. Electric potential values 759.52 mV to 731.40mV for Si based PZR sensor and electric potential values 604.43 mV to 569.27mV for SOI based PZR sensor and also electric potential values 2565.84 mV to 2570.96mV for SiC based PZR sensor When temperature ranges from 0C and 1000C for applied pressure of 1MPa. From the observation, as temperature increases the electric potential starts to decrease linearly. Table 3.7 shows electric potential values for different temperatures

   Fig 3.6. Temperature v/s Electric Potential.

   Table3.7 . Temperature Sensitivity analysis of Silicon, SOI and SiC Based PZR Sensors.

   Pressure Sensors	Sensitivity (mV/K)
   Silicon based PZR Sensor
   SOI based PZR Sensor
   SiC based PZR Sensor

4. CONCLUSION

In the paper we present the analysis of junction leakage currents for Si, SOI and SiC piezoresisitve pressure sensors. The leakge current analysis of all three sensors is done and the study reveals that the SiC based piezoresistive pressure sensor shows very less temperature sensitivity with littele junction leakage current. SiC piezoresistors used are less temperature sensistive and SiO2 is used as an insulating layer between the resistors and the substrate, this further reduces the junction leakage current. Hence SiC is a formiddible replacement for the conventonal piezoresistive pressrue sensors for high temperautre applications.

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